Z. Li et al. / Catalysis Communications 51 (2014) 58–62
61
Table 1
lines of Mo2N substantially enhanced, indicating the improved crystal-
Reaction results of dehydrogenation of benzyl alcohol on the Mo2N nanobelts.
linity. However, minor diffraction lines of MoO2 were still observed
probably due to the slow diffusion of nitrogen atoms into the lattice of
molybdenum oxides. At 850 °C, the diffraction lines of MoO2 entirely
disappeared and these of Mo2N solely presented. These results evidence
that the transformation of α-MoO3 to Mo2N through a partially reduced
MoO2-like intermediate and a higher temperature up to 850 °C may be
required to produce pure Mo2N. The Mo2N nanobelts had a width
of about 60 nm and lengths of 0.5–7.2 μm in the temperature range
examined, similar to the belt-shaped α-MoO3 precursor. This observa-
tion is consistent with previous studies on the evolutions of crystal
phase and morphology during nitridation of MoO3 with ammonia [16,
24]. For example, MoO2 and Mo2N were simultaneously formed at
500–600 °C, whereas Mo2N was exclusively produced only at 700 °C
[16]. Nitridation of MoO3 or (NH4)6Mo7O24·4H2O with ammonia at
625–750 °C produced molybdenum nitrides, but their size and mor-
phology were closely related with the molybdenum precursor. It is gen-
erally believed that such a pseudomorphic transformation involved a
reaction mechanism in which the molybdenum atoms are constrained
[24]. Here, similar phase transformation was observed during the
nitridation of the belt-shaped α-MoO3 precursor with ammonia, in
which the MoO2-like compound acted as the key intermediate for the
formation of the face-centered cubic molybdenum nitrides.
Fig. 3 shows the XPS profile of Mo 3d in the Mo2N nanobelts.
The percentage of surface Mo2+, Mo4+, and Mo5+ species was up to
74.2%, suggesting that these coordinatively unsaturated molybdenum
species are predominant on the surface of the Mo2N nanobelts. This is
quite similar to the early observations on bulky molybdenum nitrites.
The dominant molybdenum species on a Mo2N sample (4 m2/g) were
Mo2+ (57%) and Mo4+ (29%) [7]. Another Mo2N sample (140 m2/g)
presented Mo2+ (70%) and Mo4+ (23%) species on its surface [16].
The Mo2N nanobelts effectively catalyzed dehydrogenation of ben-
zyl alcohol with N99% selectivity towards benzaldehyde. As listed in
Table 1, the conversion of benzyl alcohol, on the Mo2N nanobelts, was
32% within 24 h at 120 °C. As the temperature was raised to 150 °C,
the conversion of benzyl alcohol reached 18% at 2 h, gradually increased
over time, and approached 100% at 28 h. A blank test without the use of
catalyst did not produce any benzaldehyde, indicating the heteroge-
neous catalysis nature of the reaction. Recycle tests verified that the
Mo2N nanobelts retained stable activity and selectivity for three consec-
utive runs, demonstrating that the Mo2N nanobelts are a promising
catalyst for the dehydrogenation of benzyl alcohol.
Entry
Temperature (°C)
Time (h)
Conversion (%)
1
2
3
4
5
6
7
120
150
150
150
150
150
150
150
150
150
24
2
4
32
18
25
32
52
90
100
100
100
–
6
12
24
28
28
28
24
Resue 1
Reuse 2
8a
Reaction conditions: 1 mmol benzyl alcohol, 4 ml dimethyl sulfoxide, and 60 mg catalyst.
a
Blank test without the use of catalyst.
100% at 21 h. It seems that these electron-donating groups in the benzyl
ring of the alcohols favored a relatively higher dehydrogenation activity
on the Mo2N nanobelts. For comparison, dehydrogenation of other alco-
hols was also tested. Secondary aromatic alcohol, 1-phenylethanol, was
dehydrogenated to acetophenone with a yield of 45% at 2 h and 100% at
24 h. In the case of cycloaliphatic alcohols, like cyclohexylmethanol,
however, the yield of cycloaldehyde was only 11% within 24 h. Howev-
er, the Mo2N nanobelts showed lower activities in the dehydrogenation
of secondary aliphatic alcohols like 2-octanol. The reaction proceeded
very slowly and the conversion of 2-octanol was only 6% at 24 h. All
these results evidenced that the Mo2N nanobelts were more active for
the dehydrogenation of aromatic alcohols, but much less active for ali-
phatic alcohols.
To date, only a few heterogeneous catalysts, mostly precious metal
nanoparticles, have been demonstrated to be highly active for the non-
oxidative dehydrogenation of alcohols, but their selectivities towards
the desired products strongly depended on the acid–base properties of
the oxide-support. For example, Au nanoparticles supported on a verity
of oxides offered 2.1–89% conversion of benzyl alcohol and 49–99%
selectivity towards benzaldehyde at 120 °C for 6 h; hydrotalcite-
supported gold catalysts provided the highest activity and selectivity
in the non-oxidative dehydrogenation of alcohols [2]. The superior per-
formance was primarily ascribed to the basic sites on the hydrotalcite,
which extracted a proton from the hydroxyl group of alcohol and then
underwent α-hydride elimination to yield the carbonyl product [25].
Ag nanoparticles on alumina were also active for the dehydrogenation
of alcohols; but the yields of aldehydes varied largely from 16% to 94%;
the Al2O3 support provided basic sites for the abstraction proton from
alcohol and acidic sites for the release of hydrogen, while the silver
site was responsible for the dissociation of the C\H bond [4]. Pt
nanoclusters on Al2O3 were highly active for the dehydrogenation of
The Mo2N nanobelts were also active for the dehydrogenation of
other types of aromatic alcohols. As shown in Table 2, dehydrogenation
of 4-methoxybenzyl alcohol exclusively produced 4-methoxybenzyl
aldehyde; the conversion of the substrate reached 26% at 2 h, increased
to 49% at 6 h, and approached 100% at 20 h. A similar reaction pat-
tern was observed in the dehydrogenation of 4-methybenzyl alcohol
to 4-methylbenzyl aldehyde; the conversion of the substrate reached
Table 2
Reaction results of dehydrogenations of alcohols on the Mo2N nanobelts.
Substrate
Product
Time (h) Conversion (%)
4-Methoxybenzyl alcohol 4-Methoxybenzyl aldehyde
2
4
26
38
6
49
20
2
4
6
21
2
100
20
34
44
100
45
4-Methybenzyl alcohol
1-Phenylethanol
4-Methybenzyl aldehyde
Acetophenone
24
100
11
Cyclohexylmethanol
2-Octanol
Cyclohexanecarboxaldehyde 24
2-Octanone 24
6
Fig. 3. XPS profile of Mo 3d in the Mo2N nanobelts.
Reaction conditions: 1 mmol alcohol, 4 ml dimethyl sulfoxide, 60 mg catalyst, and 150 °C.